Blueschist

Blueschist ( /ˈbluːʃɪst/), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and similar rocks at relatively low temperatures (200–500 °C (392–932 °F)) but very high pressure corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite. This combination of low temperature occurring at significant depth can only be explained in the context of plate subduction, followed by exhumation, which accounts for the rarity of this rock.

Blueschists are schists typically found within orogenic belts as terranes of lithology in faulted contact with greenschist or rarely eclogite facies rocks.